1. Field of Invention
The present invention relates to a rear derailleur, and more particularly to a rear derailleur for bicycle, wherein when a transmission lever is moving back and forth in a threaded shaft base, a foot end of a lever control cable pulls a chain lever to rotate, so that by means of a retracting spring, a distal point of the chain retaining lever can be guided to different positions. Thus, effects such as gear shift-up, gear fixing, gear shift-down, and angle adjusting of chain retaining steering wheel can thus be achieved. In addition, the operation of gear shifting is accurate and the slipping off of the chain is also prevented.
2. Description of Related Arts
Riding a bicycle has been developed from being a traditional transportation in the past to a bicycle-racing sport nowadays. People normally treat bicycle riding as a physical exercise for fitness and thus a derailleur system is introduced for making the riding more comfortably and faster in different riding conditions. A conventional derailleur system usually utilizes a transmission cable to tighten or release the derailleur. A lever and a spring of the cantilever type derailleur drive an activating lever to move, so that a chain which is engaged with a chain retaining steering wheel moves on a cassette so that different types of cassette crankset can be selected. And when a difference of the numbers of the sprockets between the front cranked and the rear cassette is minimized, less force is required to be applied on the pedal. Conversely, more effort is needed.
Currently, a rear derailleur usually is a cantilever-spring type derailleur that mainly has two types, i.e. a positive direction type and a negative direction type. An operation of a positive direction type derailleur is described as follows: when the derailleur is not in operation (not pulled by the transmission cable), the activating lever of the chain retaining steering wheel is at a position corresponding to the smallest chain wheel of the cassette (with the fewest number of sprockets). When the derailleur is in operation (the transmission cable is being pulled) to shift up, the activating lever of the chain retaining steering wheel moves towards a bigger chain wheel of the cassette, wherein the spring retained at the lever is becoming tightened when shifting up to a bigger chain wheel of the cassette (the pedaling force required is becoming less), for the difference of numbers of sprockets between the front crankset and the rear cassette being reduced. When the derailleur is shifting down (the transmission cable is being released), the spring is gradually returning to its original position (force free). Through a retracting resilient force of the spring, the activating lever of the chain retaining steering wheel moves towards to a smaller chain wheel of the cassette when the derailleur is shifting down (more and more pedaling force is becoming required).
A negative direction type derailleur is also developed, wherein its operation is opposite to the operation of the positive direction type deraillwur. When the derailleur is in operation (the transmission cable is being pulled), the activating lever of the chain retaining steering wheel moves towards a smaller chain wheel of the cassette, and conversely, when the transmission cable is being released, the activating lever of the chain retaining steering wheel moves towards to a bigger chain wheel of the cassette. Thus, it is effort-saving for the bicycle to climb on a slope. No matter the cantilever-spring type rear derailleur is in positive direction or negative direction, there is a common disadvantage that the lever and spring are exposed and easy to carry dirt and sand during operation, and even grass weeds may attach thereto, so that it is not only difficult to clean, but also may result in a disorder of the derailleur system and cause a slipping off of the chain. Thus the conventional derailleur is not convenient for use and the applicant herein provides a novel rear derailleur to achieve effects such as gear shift-up, gear fixing, gear shift-down, and angle adjusting of chain retaining steering wheel. The occurrence of disorder of the derailleur system and slipping off of the chain are also reduced.